This invention relates generally to control of the flow of refined metal in an ESR-CIG apparatus and more specifically to a CIG apparatus providing a more efficient and controlled flow of liquid refined metal. The ESR apparatus is an electroslag refining apparatus and the CIG is a cold walled induction guide apparatus. More particularly the invention relates to controlling the flow of liquid metal as a liquid metal stream to, through and from a CIG. Such liquid metal flow may be used in conjunction with nucleated casting for large metal ingots used in articles of manufacture, such as turbine wheels.
Electroslag refining (ESR) is a process used to melt and reline a wide range of alloys for removing various impurities therefrom. Typical alloys, which may be effectively refined using electroslag refining, include those based on nickel, cobalt, or iron. The initial, unrefined alloys are typically provided in the form of an ingot which has various defects or impurities which are desired to be removed during the refining process to enhance metallurgical properties, including oxide cleanliness, grain size and microstructure, for example.
In a conventional electroslag apparatus, the ingot is connected to a power supply and defines an electrode, which is suitably suspended in a water-cooled crucible containing a suitable slag corresponding with the specific alloy being refined. The slag is heated by passing an electrical current from the electrode through the slag into the crucible and is maintained at a suitable high temperature for melting the lower end of the ingot electrode. As the electrode melts, a refining action takes place with oxide inclusion in the ingot melt being exposed to the liquid slag and dissolved therein. Droplets of the ingot melt fall through the slag by gravity and are collected in a liquid melt pool at the bottom of the crucible.
The refined melt may be extracted from the crucible by a conventional induction-heated, segmented cold-walled induction heated guide (CIG). The refined melt extracted from the crucible in this matter provides an ideal liquid metal source for various solidification processes including spray deposition.
The electroslag apparatus may be conventionally cooled to form a solid slag skull on the surface for bounding the liquid slag and preventing damage to the crucible itself as well as preventing contamination of the ingot melt from contact with the patent material of the crucible. The bottom of the crucible typically includes a water-cooled, copper cold hearth against which a solid skull of the refined melt forms for maintaining the purity of the collected melt at the bottom of the crucible. The CIG discharge guide tube or downspout below the hearth is also typically made of copper and is segmented and water-cooled for also allowing the formation of a solid skull of the refined melt for maintaining the purity of the melt as it is extracted from the crucible.
The cold hearth and the guide tube of the conventional electroslag refining apparatus are relatively complex in structure, and are therefore expensive to manufacture. The guide tube typically joins the cold hearth in a conical funnel with the induction heating coils surrounding the outer surface of the funnel and the downspout through which the liquid metal flows.
A plurality of water-cooled induction heating electrical conduits surround the guide tube for inductively heating the melt for controlling the discharge flow rate of the melt through the tube. Alternating currents in the induction heating electrical conduits surrounding the copper funnel segments induce alternating eddy currents within the copper segments. In turn the alternating eddy currents within the copper funnel segments of the guide tube induce currents within the liquid metal in the flow path through the guide tube.
FIG. 1 illustrates a system 5 for nucleated casting of liquid metals. The system includes a refining system 10, a pouring system 60, and a spraying system 80, which are described below. FIG. 1 illustrates the refining system 10 for refining alloy metals in an electroslag refining furnace. Referring to FIG. 1, at the top is the melting system, which is essentially a short electroslag refining furnace 15. A consumable electrode 20 is fed into the electroslag refining furnace 15 from above using a drive mechanism (not shown). The bottom face 25 of the consumable electrode 20 is immersed into a hot liquid slag 35, which heats the bottom face 25 of the electrode 20 causing it to melt. Metal droplets are formed on the face of the electrode and fall through the slag 35 to form a liquid metal pool 40 below the slag 35. Any oxide inclusions that are present in the electrode 20 will be exposed to the slag and will be dissolved. The slag 35 is kept hot with alternating electric current 46 from the consumable electrode power supply 45, generally at low voltages and conventional frequencies, that is fed into the slag through the consumable electrode 20. The required voltage is measured as a signal that is used to control the rate of advance of the consumable electrode 20 as the bottom face 25 is melted. An unconsumable electrode 50 is also shown, being an upper portion of the ESR crucible 55. Then electric current 47 may be fed from power supply 70 into the unconsumable electrode 50 instead of, or in addition to the current supplied to the consumable electrode 20.
A pouring system 60 provides for bottom pouring from the ESR furnace 15 to form the liquid metal stream 30. To avoid contaminating the liquid metal stream 30 with oxide inclusions that may erode from a ceramic nozzle, a CIG 65 with a ceramic-free induction-heated copper funnel 61 is used to form the liquid metal stream 30.
The copper funnel 61 may be segmented radially and surrounded by one or more induction coils 66, 67. The electric current is oscillated in the induction coils 66, 67, inducing a current in each of the copper segments, and subsequently inducing a heating current in the flowing liquid metal stream 30. Heat that is induced in the copper components is removed with cooling water flow 63.
In some such conventional CIG systems, the power may be delivered to each of the induction coils at different frequencies. The amount of power delivered to each of the induction coils and the cooling water to cool the copper funnel 61 may be controlled to start and stop the flow of liquid metal in the nozzle, the amount of superheat supplied, and the volumetric rate of flow.
A CIG 90 with conventional copper funnel 91 such as from U.S. Pat. No. 5,160,532 by Benz et al. is illustrated in cutaway FIG. 2. The funnel 91 is composed of multiple copper segments 92 that are radially distributed around a central axis 93. Induction coils 94 are mounted on the underside of the funnel 91. The copper segments 92, known as copper fingers, are mechanically supported at an outer radial end by baseplate 95 or other structures of the CIG 90. Cooling water may be provided to the CIG through channels 96 providing supply and return ducts 97 to the individual copper segments 92. Separate layers of electrical insulation 98 have been applied between copper segments 92. Utilization of large numbers of copper segments, however, have resulted in structurally inadequate finger structures in contact with the liquid metal flow, thereby causing mechanical stability issues and lack of control related to varying of the hole size for the liquid metal flow. Experimentation with CIGs using such segmented copper funnels has shown the device to produce undesirably low efficiency.
An insulator is a material or object that prevents the flow of electrical charges, thereby preventing the flow of an electric current. While an electrical insulating material must be capable of withstanding the voltage and frequency of the power source which they are intended to insulate, the material must also be suitable for environment in which is to operate. These environmental factors include temperature, mechanical wear, and chemical composition of the surroundings. Further, while maintaining the appropriate electrical insulating protection characteristics, the insulating material must also not adversely impact other materials or components to which it comes in contact or to which it is exposed. Exposure to harsh environments requires insulating materials that can withstand the environment. Such a harsh environment is encountered in metal refining processes.
No electrical insulation has been employed between the copper segments and the liquid metal pool (not shown) within the funnel 92, owing to the harsh environment. Conventional electrical insulators cannot withstand the harsh environment of this application. Other unconventional insulation, such as plasma sprayed alumina, is thick and friable. Such insulators crack or crumble when in contact with the refined flow of the liquid metal and therefore are unacceptable for use because they introduce the insulating material as an impurity into the refined metal.
However, unless the copper segments of the guide tube are electrically insulated from the liquid metal, some of the induced currents within the copper segments of the guide tube will flow into the liquid metal, thereby reducing the transfer of energy through induction into the liquid metal. Therefore, it is desirable to electrically insulate the copper segments of the guide tube from the liquid metal flowing through the guide tube. The insulating layer on the copper segments must sustain high thermal gradients and thermal shock imposed during the heating and cooling of the liquid metal. The insulating layer must be robust, but at the same time thin so as not to interfere with the liquid metal flow taking place in a specially shaped flow path of the funnel.
Again referring to FIG. 1, an atomization and collection system 80 is also part of such a casting system. After a short free-fall from the CIG 90, the liquid metal stream 30 is atomized using a conventional open atomizer 81. The atomizer 81 directs a gas jet onto the liquid metal stream 30 and converts it into a spray 83, accelerating the spray droplets from the atomization zone toward a collection mold 85, cooling them in flight.
Other collection systems may be employed including, but not limited to, metal powder atomizing, melt spinning, spray forming, nucleated casting, direct casting, etc.
Accordingly, there is a need to provide a more efficient and robust cold induction guide for a nucleated casting process. One aspect of the improved efficiency is the need for an electrical insulating material for the cold-walled induction-heated guide tube which electrically isolates the induction currents in the guide tube from leaking into the stream of liquid metal passing through the guide tube, but which does not contaminate the liquid metal being processed.